Machines such as, for example, on-highway vocational vehicles, off-highway haul trucks, wheel loaders, motor graders, and other types of heavy machinery are used for a variety of tasks. These machines generally include a power source, which may embody, for example, an engine, such as a diesel engine, a gasoline engine, or a gaseous fuel-powered engine that provides the power required to complete these tasks. The power produced by the power source may be transmitted through a transmission, such as, for example, a continuously variable transmission (“CVT”), to one or more ground engaging devices or traction devices in order to propel the machine.
Machine control systems are often used to coordinate and regulate operation of the power source and CVT to improve the machine's responsiveness and efficiency. For example, while the machine is traveling the power source and CVT may have a range of speeds and torques at which the power source and CVT experience substantially stable and efficient operation. Operating outside of this range may increase fuel consumption and/or decrease responsiveness.
Machine velocity control schemes may utilize the reading of a speed pedal's displacement to create a jerk command. Jerk is defined as the second derivative of velocity and the first derivative of acceleration (or rate of change of acceleration). A high jerk value provides aggressive machine operation, however, it could cause the operator to lose control due to the rapid movement of the machine by creating unwanted speed pedal modulation. On the other hand, a low jerk value will provide time for the operator to react to unwanted speed pedal modulation. However, it can result in a relatively sluggish operation. The application of jerk has a linear slope so that the amount of jerk is constant throughout the full range of pedal displacement. A constant jerk value is a compromise that attempts to prevent either an overall sluggish operation or too much unwanted speed pedal modulation. Unfortunately, this results in too much jerk being present when the operator desires a greater degree of control and not enough jerk being present when the operator desires quick machine response.
One method for controlling a power source and CVT is disclosed in U.S. Pat. No. 7,192,374 (the '374 patent) issued to Kuras et al. on Mar. 20, 2007. The '374 patent discloses an engine underspeed control system that adjusts the transmission ratio so that the engine is running at an optimal speed condition (i.e., within a range of speeds where the engine is operating most efficiently). The control system of the '374 patent discloses an operator input that provides an input signal to a controller. The operator input, for example, could be an accelerator pedal that allows the operator to depress the pedal to request an increase in machine output speed. The input signal may represent a requested speed, which the controller then converts into a motor speed command (the motor being a component of a hydraulic CVT that is powered by an engine). The control system of the '374 patent prevents the motor speed command from exceeding an upper speed limit and from dropping below a lower speed limit. These limits are calculated such that, as long as the motor speed command remains within the upper and lower speed limits, the motor torque command will stay within the torque capability of the motor. The motor torque limit at a particular motor speed can be determined from the torque-speed curves for the motor. The engine underspeed control algorithm (implemented by the controller) will also reduce the motor speed command if the engine begins lugging (e.g., if the engine speed drops below a threshold value). The method of the '374 patent thus enables the CVT to respond quickly to changes in the motor speed command while preventing damage to the motor and transmission.
Although the machine of the '374 patent may help the motor remain responsive while preventing potential damage to the motor and transmission, it may not provide for the level of responsiveness desired by an operator under all conditions. By only controlling the motor speed, the control system of the '374 patent may allow the engine to operate at an inefficient and/or unresponsive engine speed (i.e., either too low or too high), or may result in an undesirable level of aggressiveness of response under certain operating conditions.
The disclosed propulsion control system is directed to overcoming one or more of the problems set forth above.